1. Field of the Invention
The present invention relates to the field of electronic circuits used in data transmission, and more particularly a receiver for an integrated heterodyne communication system including BAW type resonators.
2. Description of the Related Art
The heterodyne architecture was developed in the beginning of the XXth century for the purpose of improving wireless communication. Such architecture is still used in the more recent mobile receivers: mobile telephones, so-called pagers, 433 Mhz receivers etc. . . .
The principle of the heterodyne architecture is shown in FIG. 1, where a low noise amplifier 12 receives a Radio Frequency (RF) signal generated by an antenna 11 and operating at a frequency fRF. A filtering element 13 is inserted between amplifier 12 and a frequency mixer 14. The frequency mixer brings back the RF signal to another frequency which is generally designated by the expression Intermediate Frequency fIF, thanks to a mixing with a reference signal fOL generated by a local oscillator (LO). This permits the signal to be processed at the Intermediate Frequency fIF.
Assuming that fOL<fRF, the following relation is true: fRF=fOL+fIF. The same reasoning could be followed for the case where fOL<fRF.
Because the receiving antenna is a relatively wide band tuned circuit, other undesired interfering signals are inserted in the input of the LNA circuit, and particularly at the image frequency fimage=fOL−fIF.
Generally speaking, it is necessary to remove this image frequency which might interfere with the receipt of the RF signal. Practically, three methods are known for achieving such suppression.
A first method is based on the use of a filter for suppressing the image frequency, as represented by element 13 of FIG. 1. This filter is designed for providing a transfer function which lets all the signals go through at the exception of the image frequency which is substantially reduced. The expression “notch-filter” can refer to such method and, by extension, a heterodyne architecture which is based on this method of rejection is referred to as a Notch type architecture.
A second known method consists in using, not a notch filter, but a narrow band filter for leaving only the useful signals. With this second method, which is certainly better than the first method, all the non-useful signals, including the signals at the image frequency, are rejected. In practice, this method and the architecture using such method—which is generally designated by the expression “narrow band architecture” is a delicate task to achieve because of the difficulty of realizing a narrow band filter showing performance. And much more tricky to integrate in a semiconductor circuit.
In order to compensate for the above mentioned difficulty, a third method can be used or, should we say, a third category of methods which include numerous processing operations which permits to get rid with the problem of the image frequency. For instance, the homodyne type architecture is based on a direct conversion of the RF frequency. This third category further includes architectures of the type Low-IF involving a complex frequency processing, and also Hartley or Weaver type methods which are sometimes referred to as IQ architecture.
Generally speaking, the above evoked methods are discussed in the following document:    Aarno Pärssinen, ‘Direct Conversion Receivers in Wide-band Systems’, Kluwer Academic Publishers, 2001, which is incorporated by reference herein.
Now considering the case of the mobile telephone of the type Wide Band Code Division Multiplexing Access (WCDMA), the following frequencies are used:
for receiving: a 60 MHz frequency range between 2110 Mhz and 2170 Mhz;
for emitting: a 60 MHz frequency range between 1920 Mhz and 1980 Mhz.
There is a 5 Mhz frequency range which is assigned to one channel. The processing of one channel, in a known heterodyne type architecture, requires to suppress all non-desired signals, and particularly the image frequencies and, in addition, other high level signals at determined frequencies which are often referred to as “blocking” signals.
All these signals must be rejected or at least significantly reduced before they reach the mixer 14.
In most circuits of known WCDMA type mobile telephone, there is used an appropriate selective band pass filter which is located at the input of the mixer and realized by means of acoustic components of the type Surface Acoustic Wave (S.A.W.). These components have the drawback of not allowing easy integration in a same semiconductor product.
Downstream of this SAW filter, there is used a very selective filtering circuit for the purpose of suppressing the image frequency with a high level of rejection, generally at least 50 dB.
In the WCDMA standard, for instance, the band for the emission is 60 MHA and is divided into channels of 5 Mhz. The separation of these channels requires the use of selective filters fitted with quality factors having a value of at least several hundreds.
For W-CDMA transmission: fcenter=2140 MHz, Bp—channel=5 MHz.Q=fcenter/Δf|−3dB≈fcenter/Bp—canal=428.
For GSM transmission: fcenter=943 MHz, Bp—channel=0.2 MHz.Q=fcenter/Δf|−3dB≈fcenter/Bp—channel=4715.
The figures mentioned above are a clear obstacle preventing the use of the second method which was discussed before, because of the great difficulty of realizing filters for separating channels providing high selectivity at the radio frequency and, moreover, for providing a complete integration within a same semi-conductor product.